Cellular Reproduction

It may appear that little is happening in the cell during the gap phases. Most of the activity occurs at the level of enzymes and macromolecules. The cell produces things like nucleotides for synthesizing new DNA strands, enzymes for copying the DNA, and tubulin proteins for building the mitotic spindle. During the S phase, the DNA in the cell doubles, but few other signs are obvious under the microscope. All the dramatic events that can be seen under a microscope occur during the M phase: the chromosomes move, and the cell splits into two new cells with identical nuclei.

Mitosis

Mitosis is a form of cell division where two identical nuclei are produced from one nucleus. DNA contains the genetic information of the cell. It is stored in the nucleus. During mitosis, DNA in the nucleus must be copied, or replicated. Recall that this happens during the S phase of the cell cycle. During the M phase, this copied DNA is divided into two complete sets, one of which goes to a daughter cell.

When DNA replicates, it condenses to form chromosomes that resemble an X. The DNA forms chromosomes by wrapping around proteins called histones. As shown in the image, it takes two identical sister chromatids to form a chromosome. A centromere holds the sister chromatids together.

Four phases take place during mitosis to form two identical daughter cells:

1. Prophase: The nuclear membrane disappears, and other organelles move out of the way. The spindle, made of microtubules, begins to form. During prometaphase, the microtubules begin to attach to the centromeres at the center of the chromosome.

2. Metaphase: Spindle fibers line the chromosomes at the center of the cell. This is because they are pulled equally by the spindle fibers, which are attached to the opposite poles of the cell.

3. Anaphase: The chromosomes are pulled to the opposite poles of the cell.

4. Telophase: The chromosomes de-condense, the nuclear membrane reappears, and other parts of the cell return to their usual places in the cell.

The cell divides into two daughter cells by way of cytokinesis.

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Test Tip

There is a popular mnemonic to help remember the order of the phases for mitosis:
[Please] Pee on the MAT
The “please” refers to prophase, while “pee” refers to prometaphase. MAT stands for metaphase, anaphase, and telophase, respectively.

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Meiosis

Meiosis, sexual cell division in eukaryotes, involves two phases of mitosis that take place one after the other but without a second replication of DNA. This provides the reduction in chromosome number from 2n  to n  needed for fertilization to restore the normal 2n  state. 

Diploid multicellular organisms use meiosis, which reduces the number of chromosomes by half. Then, when two haploid (n) sex cells (sperm, egg) unite, the normal number of chromosomes is restored. Diploid organisms, such as humans and oak trees, have two copies of every chromosome per cell (2n), as opposed to n, when one copy of every chromosome is present per cell.

Keep In Mind

During prophase I of meiosis, crossing over occurs to increase genetic diversity. Corresponding chromosomes from the mother and the father of the organism undergoing meiosis are physically bound, and X-shaped structures called chiasmata form. These are where corresponding DNA from the different parental chromosomes are exchanged, resulting in increased diversity.

The process of meiosis is divided into two rounds of cell division: meiosis I and meiosis II. 

Cytokinesis looks different in plant and animal cells. Plant cells build a new wall, or cell plate, between the two cells, while animal cells split by slowly pinching the membrane toward the center of the cell as the cell divides. Microtubules are more important for cytokinesis in plant cells, while the actin cytoskeleton performs the pinching-off operation during animal cytokinesis.

The phases that occur in mitosis (prophase, metaphase, anaphase, and telophase) also occur during each round of meiosis. Also, cytokinesis occurs after telophase during each round of cell division. However, DNA replication does not happen when meiosis I proceeds to meiosis II. The result of meiosis is one diploid cell that divides into four haploid cells, as shown in the following images.

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Key Point

Meiosis and mitosis both require cytokinesis to physically separate a cell into daughter cells. Also, the sequence of events that occur in mitosis are the same in meiosis. However, there are two primary differences between the types of cell division: (1) meiosis has two rounds of cell division, and (2) daughter cells are genetically identical to the parent cell in mitosis but are not genetically identical in meiosis.

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Let’s Review

• Cells are needed for growth, repair, and reproduction. Living things can reproduce asexually or sexually.
• Prokaryotes and some eukaryotes reproduce asexually, a process where a single parent cell copies its DNA and splits in half to reproduce. All offspring are genetically identical.
• Sexual reproduction is a process in which two gametes, otherwise known as reproductive cells, unite to create offspring with genetic characteristics from both parents, providing greater genetic diversity in the population.
• A chromosome is a rod-shaped structure that forms when a single DNA molecule and its associated proteins coil tightly before cell division.
• The cell cycle regulates cellular division and can be divided into two phases: interphase and the M (mitotic) phase. The cell cycle consists of a G1 (growth) phase, S (DNA replication) phase, G2 (preparation) phase) and the M (mitotic) phase.
• Mitosis is a form of cell division where one parent cell divides into two identical daughter cells. It primarily consists of prophase, metaphase, anaphase, and telophase.

  • Prophase: Chromosomes condense into two identical sister chromatids attached by a centromere for division.
  • Metaphase: Chromosomes align at the center of the cell.
  • Anaphase: Chromosomes are pulled apart and sister chromatids are pulled to opposite poles of the cell by spindle fibers.
  • Telophase: Chromosomes de-condense and a nuclear membrane appears. The formation of a cell wall to divide the cell into two daughter cells occurs by cytokinesis.

• Meiosis is a different cellular division process that is divided into two rounds of cell division: meiosis I and meiosis II.
• Meiosis I and Meiosis II involve the same phases as mitosis. The major difference is that DNA replication does not happen when meiosis I proceeds to meiosis II.
• Prophase I of meiosis includes a process of crossing over. This process increases genetic diversity because corresponding DNA from different parental chromosomes is exchanged.
• The products of meiosis are four haploid cells, or cells with half the number of normal chromosomes.
• The normal number of chromosomes is restored when two haploid sex cells (sperm, egg) unite, forming a zygote, or diploid cell with a normal number of chromosomes.

Cellular Reproduction Flashcards